I just spoke to a briefer at the Terre Haute Flight Service Station, and after looking it up in a book she gave me the following definition of a "heavy" aircraft: "An aircraft that is capable of operating at a weight of 255,000 pounds, whether or not it is doing so on a particular flight." Therefore, the 757 is not a heavy, because even it's max. taxi weight is <255,000 lbs, but those aircraft that are heavy are always heavy, regardless of load. i.e. AA's 777 would have been identified as heavy when it was doing touch and goes in Fort Worth for pilot training before entering service, despite the fact that it probably had relatively little fuel and only a few people on board.